A) Field of the Invention
The present invention relates to a compound semiconductor device and a Doherty amplifier using the compound semiconductor device, and more particularly to a Doherty amplifier using field effect transistors, and a compound semiconductor device applicable to a peak amplifier of the Doherty amplifier.
B) Description of the Related Art
Developments are made vigorously in an electronic device which has crystalline AlGaN/GaN grown on a substrate made of sapphire, silicon carbide (SiC), gallium nitride (GaN) or silicon (Si), and uses a GaN layer as an electron transport layer (e.g., JP-A-2006-165207). GaN has a band gap of 3.4 eV and is expected to operate at higher breakdown voltage than GaAs having a band gap of 1.4 eV. If a high electron mobility transistor (HEMT) made of GaN and having a high breakdown voltage is used as an amplifier, this amplifier can operate along a load line corresponding to large load impedance on a graph showing current-voltage characteristic. A high efficiency operation is therefore possible.
A base station amplifier for world interoperability for microwave access (WiMAX) requires a nonconventional high efficiency. In order to realize a high efficiency, use of Doherty amplifiers has been studied (For example, JP-A-2006-166141).
FIG. 1 is a fundamental equivalent circuit diagram of a Doherty amplifier. A high frequency signal input from an input terminal Ti is divided into two parts. One part is input to a carrier (main) amplifier 100 and the other part is input to a peak (auxiliary) amplifier 101 via a quarter-wave line 103. Another quarter-wave line 102 is connected to an output terminal of the carrier amplifier 100. An output signal of the carrier amplifier 100 passes through the quarter-wave line 102 and thereafter is combined with an output signal of the peak amplifier 101. A load impedance RL is connected to an output terminal To. The carrier amplifier 100 is biased to operate as class A or class AB, and the peak amplifier 101 is biased so that an idle current is smaller than that of the carrier amplifier 100.
FIG. 9 shows an example of input/output characteristics of a Doherty amplifier. In FIG. 9, a solid line ac and a broken line ap0 indicate the input/output characteristics of the carrier amplifier 100 and peak amplifier 101, respectively. A solid line at indicates the input-output characteristics of the Doherty amplifier including the carrier amplifier 100 and peak amplifier 101.
When an input power is small and the Doherty amplifier operates in the back-off region, mainly the carrier amplifier 100 operates and outputs an output signal. When the input power is sufficiently large and an output power of the carrier amplifier 100 saturates, the peak amplifier 101 operates and outputs an output signal. During the operation in the back-off region, a d.c. consumption power of the peak amplifier 101 is sufficiently small. An efficiency of the Doherty amplifier itself is therefore high. Since output signals of the carrier amplifier 100 and peak amplifier 101 are combined, a large output power can be obtained.
It has been found that if a HEMT using GaN are applied to the carrier amplifier 100 and peak amplifier 101 of the Doherty amplifier, an efficiency of the Doherty amplifier is not improved so much as expected. This is because even if an idle current of the peak amplifier 101 is made small, a gain profile of the peak amplifier 101 is similar to that of the carrier amplifier 100. With similar gain profiles of both of the amplifiers, even if the Doherty amplifier operates in the back-off region, a gain of the peak amplifier 101 is large and a d.c. component of an output signal of the peak amplifier 101 is large. The efficiency is therefore not improved so much as expected.
While the carrier amplifier 100 operates in a saturated state, a Schottky barrier between a gate electrode and a substrate lowers, and a forward gate leak current increases. If the Doherty amplifier has distortion compensation, this distortion compensation is disabled by the gate leak current.